1. The Field of the Invention
The present invention relates to a construction of a multi-phase flat-type PM (Permanent Magnet) stepping motor and a driving circuit thereof. Particularly, the present invention relates to an improvements of a high-resolution and high-accuracy PM stepping motor and a driving circuit thereof that are suitable for OA (Office Automation) equipment, which requires accurate positioning during high speed operation, such as a printer, a high speed facsimile or a PPC copying machine.
2. Prior Art
FIG. 21 is a longitudinal sectional side view of one example of this kind of conventional multi-phase flat-type PM stepping motor (referred to as a xe2x80x9cmotorxe2x80x9d in the following description), and FIG. 22 is a front view of the main portion from a XXIIxe2x80x94XXII line in FIG. 21.
In the drawing, a reference 101 denotes a stator, 102 denotes air-core coils that are radially arranged and 3 denotes a magnetic disc on which permanent magnets 4 are attached. The magnetic disc 3 is fixed to a rotation axis 8, and this rotation axis 8 is rotatably supported by bearings 7 fixed to the stator 101 through brackets 1B. The permanent magnets 4 alternatively magnetized in N-pole and S-pole that are radially arranged. Each of the permanent magnets 4 constitutes a magnetic pole. The pitch of the permanent magnets 4 corresponds to that of the coils 102.
FIG. 23 shows a connection example of a conventional 6-phase motor with twenty-four coils, and FIG. 24 shows a driving circuit for the connection of FIG. 23.
In FIG. 23, "PHgr"1 through "PHgr"24 denote the coils, A through F denote terminals at one end side of the coils connected in series for each of the phases and Axe2x80x2 through Fxe2x80x2 denote terminals at the other end side of the coils.
In FIG. 24, T1 through T24 are switching elements such as switching transistors to excite the respective coils, "PHgr"AAxe2x80x2 through "PHgr"FFxe2x80x2 are the coil groups in which the coils of the same phase are serially connected as shown in FIG. 23. A reference V represents a power supply.
Four switching elements form bridge connection for each phase and each terminal of the coil groups is connected to the intermediate point of the serial connection. In other words, the first switching element T1 and the second switching element T13 are connected, the third switching element T2 and the fourth switching element T14 are connected, and the terminals A and Axe2x80x2 of the coil group of the first phase shown in FIG. 23 are connected to the connection points of the switching elements.
In this connection, when the first switching element T1 and the fourth switching element T14 are conducting, an electric current passes in a direction EC1 from the terminal A to the other terminal Axe2x80x2, which energizes the coil group of the first phase. In this way, the motor rotates as the respective phases are sequentially excited by bringing the respective switching elements into conduction in order.
FIG. 25 is a connection diagram of a 10-phase motor that includes forty coils and FIG. 26 shows a driving circuit for the coils shown in FIG. 25. In FIG. 25, "PHgr"1 through "PHgr"40 denote the coils, A through T denote terminals in one end side of the coils connected in series for each of the phases and Axe2x80x2 through Txe2x80x2 denote terminals in the other end side of the coils.
In FIG. 26, T1 through T40 are switching elements such as switching transistors to excite the respective coils, "PHgr"AAxe2x80x2 through "PHgr"TTxe2x80x2 are the coil groups in which the coils of the same phase are serially connected as shown in FIG. 25. A reference V represents a power supply. Four switching elements form bridge connection for each phase, each coil group is connected to the intermediate points of the bridge connection.
In other words, the first switching element T1 and the second switching element T21 are serially connected and the third switching element T2 and the fourth switching element T22 are serially connected. The terminals A and Axe2x80x2 of the first phase coil group are connected to the connection points of the switching elements.
In this connection, when the first switching element T1 and the fourth switching element T22 are conducting, an electric current passes in a direction EC1 from the terminal A to the other terminal Axe2x80x2, which energizes the coil group of the first phase. In this way, the motor rotates as the respective phases are sequentially excited by bringing the respective switching elements into conduction in order.
A step angle is a rotation angle of one step rotation of the stepping motor when the coil groups are sequentially excited phase by phase and it is determined by the structure of the motor. It is necessary to minimize the step angle to obtain a motor having high resolution and a good control performance.
The step angle xcex8 of the multi-phase flat-type stepping motor is represented by xcex8=360xc2x0/(mxc3x97Pr), where m is phase number of the motor and Pr is a total number of magnetic poles of the rotor including N-poles and S-poles. Therefore, it is necessary to increase the phase number m or the magnetic pole number Pr in order to decrease the step angle xcex8.
In order to increase the phase number, it is required to increase the number of coils on the stator. For instance, while a 6-phase motor operates with two coils per phase (12 coils in total) in principle, the stable operation requires 24 coils. In the same manner, a 10-phase motor requires 40 coils in total.
However, since the coil has a predetermined width, when all coils are arranged in the same magnetic disc as the prior art, a number of the coil is limited, and the number of phase cannot be enough large.
On the other hand, the magnetic pole number Pr of the rotor should be increased in order to decrease the step angle without increasing the phase number. However, the magnetic pole number Pr of a rotor is fixed by precision ability of a magnetizing device and cannot be enough large.
A micro-step driving is needed to get a resolution higher than the step angle determined by the phase number and the magnetic pole number. However, since the stop position of the rotor is determined by the relative values of electric current applied to the respective phases under the micro-step driving, it was difficult to improve the accuracy of the resolution due to variation of the values of electric current applied to the respective phases, variation of characteristics of switching elements, or the like. Further, since a complicated driving circuit was need for the micro-step driving, there was a problem that the cost rises.
Further, the conventional driving circuits shown in FIGS. 24 and 26 require four switching element for each phase. Therefore, 24 switching elements are needed for driving the 6-phase motor and 40 switching elements are needed for driving the 10-phase motor. This complicates the driving circuit and increases the cost thereof.
It is the fact that the multi-phase flat-type stepping motor is hardly available in the market due to the above-described reasons.
An object of the present invention is to solve the above described problems of the conventional motor and to provide a high-resolution, high-accuracy multi-phase flat-type PM stepping motor with employing a multi-unit construction. Another object of the present invention is to provide a simple and low-cost driving circuit for the multi-phase flat-type PM stepping motor.
A multi-phase flat-type PM stepping motor of the present invention comprises a first motor unit that comprises a first stator unit and a first rotor unit, a second motor unit that comprises a second stator unit and a second rotor unit, and a non-magnetic disc arranged between the first and second motor units. The first stator unit has a plurality of air-core coils that are radially arranged on a first isolating magnetic disc. The first rotor unit has a plurality of permanent magnets that are alternatively magnetized in N-pole and S-pole and radially arranged on a second magnetic disc with a predetermined air gap with respect to the coil surface of the first stator unit. In the same manner, the second stator unit has a plurality of air-core coils that are radially arranged on a third isolating magnetic disc, and the second rotor unit has a plurality of permanent magnets arranged on a fourth magnetic disc. The second and fourth magnetic discs are fixed to a rotation axis. The first and second stator units are fixed to the different sides of the non-magnetic disc and the first and second rotor units face each other across the first and second stator units.
The coils arranged on the first stator unit may be deviated from the coils arranged on the second stator unit by xc2xd of the coil arrangement angular pitch, and the permanent magnets arranged on the first rotor unit may be deviated from the permanent magnets arranged on the second rotor unit by xc2xc of the angular pitch of the magnetic poles having the same polarity.
The total number Pr of the N-poles and S-poles of each rotor unit preferably satisfies the following equation:
Pr=mxc2x12 
where m is a phase number of the motor.
Further, the motor of the present invention may be a 6-phase motor or a 10-phase motor.
In a driving circuit for the multi-phase flat-type PM stepping motor according to the present invention, the respective opposite air-core coils are serially connected to form a plurality of coil groups, and terminals at one side of the coil groups are connected to each other for each of the first and second stator units to permit plural phase excitation.
Further, the connection point of the coil groups arranged on the first stator unit may be connected to the connection point of the coil groups arranged on the second stator unit.
Still further, terminals at the side opposite to the connection point of the coil groups may be connected to connection points of serially connected switching elements, respectively.
In the driving circuit for the 6-phase motor, the air-core coils of each motor unit may be connected as a delta connection to permit plural phase excitation.
In the driving circuit for the 10-phase motor, the air-core coils of each motor unit may be connected as a pentagonal connection to permit plural phase excitation.